The field of art to which this invention pertains is curable compositions based upon mixtures of polyepoxide resins and imidazole type curing agents. More particularly, this invention relates to liquid blends of monoimidazole curing agents and normally solid polyols used to cure liquid polyepoxides.
For many years in the epoxy plastics industry, attempts have been made to insure that two-component compositions possess a long pot life and at the same time a short cure time. In other words, it is particularly desirable that the cure response of the combined epoxide resin/curing agent mixture is such that it may be used for several hours at working temperatures without an inordinate increase in viscosity. It is also important that the polyepoxide compositions exhibit a short cure time, so that long, expensive cure schedules at elevated temperatures are not required. This combination of long pot life and short cure schedule is particularly necessary in various plastics end uses, including forming auto springs, filament wound pipe and tanks, and other fiber reinforced components. To date, no one resin system has exhibited the desired ratios of pot life and cure time.
In addition to this desired combination of cure properties, general use plastic compositions should be non-staining, of low toxicity, and in the cured state possess a high degree of chemical resistance. It is also necessary that these compositions maintain physical strength at elevated service temperatures and exhibit toughness at normal ambient temperatures.
In the prior art, solubility problems have made it extremely difficult to utilize normally solid, aliphatic polyols, particularly in 100% solids curing applications. Usually normally-solid polyols were incorporated into prior art epoxy-based compositions through the use of a solvent. However, when solvents are employed, increases in cure temperatures and times are required in order to drive the solvent off from the curing composition. In addition, as the solvent is removed during curing, voids can develop. On the other hand, previously available liquid polyols, such as ethylene glycol and glycerine, exhibit poor compatibility with most liquid epoxy systems.
In the prior art the use of imidazole type curing agents has been found to be particularly desirable where a combination of long pot life and short cure time is required. However, the incorporation of solid imidazole curing agents into epoxide compositions has been particularly difficult because of their high reactivity with heated resin. Thus, like the polyol systems in the prior art, in order to incorporate solid imidazole catalysts, it has been necessary to employ a solvent. As a result, higher curing temperatures and longer curing times have been required in order to drive the solvent off from the finally cured system. And like other prior art solvent systems, voids can develop during curing.
An alternative method of utilizing imidazole catalysts has been to dissolve them in the liquid epoxy resin at elevated temperatures. However, this procedure has resulted in greatly decreased pot lives and working times for these high temperature solubilized systems.
It is an object of this invention, therefore, to prepare liquid, non-staining, low toxicity curing agents for polyepoxides which, when blended with polyepoxides, form curable systems which exhibit increased pot lives and, at the same time, relatively short, low-temperature cure schedules.
In addition, it is an object of this invention to prepare thermoset plastic compositions with improved chemical resistance. Another object is to prepare thermoset plastic compositions which exhibit a combination of elevated temperature strength and ultimate tensile strain capability at room temperature unequalled by unmodified, catalytically cured epoxy resin plastics.
These and other objectives are obtained by preparing the compositions of the instant invention.